Electrical cooking unit



Oct. 23, 194-5. J, w MYERS ETAL 2,387,461

ELECTRICAL COOKING UNIT Filed Jan. 1, 1942 4 Sheets-Sheet l cfasep IZZYe2 Oct 1945- J. w. MYE'Rs ETAL 2,387,461

ELECTRICAL COOKING UNIT Filed Jan. 1, 1942 4- Sheets-Sheet 2 use .2262".? "la Rag-$2224.

Oct. 23, 1945. J. w. MYERS EI'AL 2,387,461

ELECTRICAL COOKING UNIT Filed Jan. 1, 1942 4 Sheets-Sheet s Oct. 23,1945. J w MYERS Er 2,387,461

ELECTRICAL COOKING UNIT Filed Jan. 1, 1942 4 Sheets-Sheet 4 J: oesci zZa 63,

21 ESQ/DA Hlf yers Patented ca. as, 1945 ELECTRICAL COOKING UNIT JosephW. Myers and Stephen J. Roesch, Philadelphia, Pa., assignor to Proctor &Schwartz,

Inc., Philadelphia, Pa.

- sy lvania a corporation of Penn- Application January 1, 1942, SerialNo. 425,330

9 Claims.

This invention relates to electrical heating units and more particularlyto surface type units such as those used on electric ranges or otherappliances. The invention further relates to ceramic type structures ofthis general character. Since the invention is particularly directed tocooking units it will be described with reference thereto.

It is the principal obiect of this invention to provide a ceramic unitof the class described which is inexpensive to manufacture, has highefiiciency and is capable of quick heating, is durable and capable ofwithstanding great thermal and mechanical abuse, and has long life.

In the past, there have been generally two types of cooking unitsemployed. The first of these is the metal sheathed unit, wherein theheating element is encased in and is insulated from a sheath of metal.These'units have enjoyed considerable popularity because of theircomparatively high efliciency and speed of operation.

They are,-however, expensive to manufacture; they have a tendency towarp under usage; and in most cases their efliciency tends to decreasewith usage as the metal reflector plate employed in conjunction withthem tamishes and becomes dull. The second type of unit is that whichemploys a heating element mounted on the face of an open refractorysupport. This type of unit is inexpensive to manufacture, but it ischar. acterized by slow and ineflicient operation and I shook hazard toan ope ator. Attempts to improve the operating characteristics of thistype of unit .by reducing the mass of the back structure have resultedin loss of mechanical strength and resistance to heat shock. I

One of the reasons for the inefficiency of the ceramic type of unitheretofore employed is that the transfer of heatto a cooking vessel iseflected mainly by thermal radiation. Prior workers in the art havetherefore suggested that the heating element be fixed to its refractorysupport by means of a thermally-conductive but electricallyinsulatingcement. By this means, it was endeavoured to increase the speed andefficiency of the unit. In addition to allowing heating of the cookingvessel by thermal conduction, a cement face is adapted to be made flatand is virtually non-warping. However, the use of cement has given riseto serious difiicultie's.

Where a heating element isembedded in a heat-conducting cement which inturn is con-- tained in or on a supporting refractory, it is obviousthat great thermal strains will tend to exist at their junction. This isnaturally due to the inevitable differences in the thermalconductivities and coefflcients of expansion between the two differentceramic bodies. This creates a serious problem as to how to prevent thecement from separating from its support and of maintaining the integrityof the structure as a whole in the face ofthermal and mechanical shock.Prior workers in the art have therefore made the heat-conducting cementof very considerable thickness and mass so that the unit waspredominately monolithic cement, and the reduced rate of heating andexpansion was not expected to exceed the ability of the support tofollowit;

or they have cemented a large coil into a-deep channel, well within thebody of a ceramic support and practicall heated the body of therefractory support and the cement as a v whole. Such structures have notbeen commercially satisfactory because of their deficiency in speed andlow operating emciency.

By the present invention, there is provided a novel cement-surfacedceramic type cooking unit in which the heating element is disposed. veryclose to the cooking surface and is distributed as uniformly as possiblein respect to said surface, thereby to obtain rapid and highly efficientoperation, and the unit is so constructed that it will not disintegrateunder the destructive forces, of severe thermal or mechanical shocks.The unit comprises a strong deep ceramic-support which carries-athinhighlyeconcentrated heating zone at 'its upper face. The heatingelement is in the form of a helical coil of smali helix ture.

diameter, disposed in shallow channelsat the face of the deep'ceramicsupport and embedded in cement which surfaces the units In operation,

' thedeep ceramic support remains at a relatively low .temperature whilethe thin heating faceof the unit' is heated to a high cooking tempera-The unitis caused to relieve itself of stresses due to unequal expansionand contraction of the cement and the ceramic support, and the integrityof the thin heating face is maintained, by initially developing amosaicfiof minute irregular cracks therein which are so imperceptible asto be unnoticeable by the naked" eye. This divides the cement face intomulti tudinous irregular small pieces which are securely locked in placeand it also provides a certain amount of flexibility in the cooking facewhich,

while remaining substantially flat, is enabled to i. shallow spiralchannel 6.

in combination with a deep strong ceramic support, the said unit beingcapable of withstanding great thermal shock.

Another object of the invention is to provide in a-unit of the classdescribed a novel arrangement for an auxiliary heating element to aid invarious cooking operations.

A further object of the invention is to provide a novel structure forconnecting the heating elements to the lead wires in a cooking unit ofthe type described.

Other objects and features of the invention will be hereinafterapparent. The invention may be clearly understood by reference to theac-- companying drawings, in which:

Fig. 1 is a perspective view of one form of lit covers the major portionof the cooking surface. In a unit of this type, of about six inchdiameter,

made for operation at about 1200 watts, the heating element 6 may beformed of Number 23 gauge wire, and may be wound in the helix on amandrel only slightly larger than its own diameter. This prcvidesaheating coil of approximately 1 6 inch cooking unit constructedaccording to the invention, with a portion of the unit broken away andwith other portions shown in unfinished state for purposes .ofillustration;

, Fig. 2 is a plan or face view of a portion of the cooking face withthe mosaic surface greatly exaggerated as to visibility of the minutecracks;

Fig. 3 is a perspective view of a preferred form of the cooking unit; I

Fig. 4 is a plan or face view of the ceramic support with the urfacingcement removed for the purpose of illustration, and showing the endportions'only of the heating elements;

Fig. 5 is a bottom view of the ceramic support showing the rib structurethereof and the' lead wires from the heating elements:

Fig. 6 is an enlarged fragmentary sectional outside diameter. Thechannels 4 are only slightly wider than the coil and may have a heightsuch that the depth of cement above the coil is about 0.040 inch. Thewidth of the ribs 5 is limited by considerations of molding and ofelectrical insulation, and may be made about inch. It will be seen thatthe source of heat in the unit is very close to the cooking surface,

view of the complete unit taken along line 6-6 of Fig.

Fig. 7 is a fragmentary perspective view of the ceramic support; and

Figs. 8 to 10 are sectional views illustrating different portions of theunit where the lead Wires are attached to the heating elements.

Referring to Fig. 1, there is shown an annular electrical cobking unitcomprising a deep ceramic support I carrying a thin highly-concentratedheating zone at its upper face. The supporting structure is a solidcircular brick of ceramic material which is provided with a plurality ofamic material may be used for the support. For

example, the ceramic material known by the trade name Thermolain issuitable.

Formed in the upper portion of brick l is a long Separating successiveconvolutions of channel'4 are rib portions 5, extending upward from andintegral with brick l. Positioned in the channel 4 is a heating element6, in the form of a helical coil of resistance wire.

also fills the radial slots 2." Heatingelement 6' is provided with acoating, preferably of enamel,

which burns off when the element is heated. An expansion space is thusprovided around the wire. This prevents the development of stresses illt e providing'greatlyincreased speed and efllciency;

This provides 'a thinhighly'heated top affixed to a relatively cool deepmassive supporting structure. As stated hereinbefore, such localizedheating of the structure gives rise to great thermal stresses which tendto disintegrate the cooking face by causing the cement to break up andbecome dislodged from the face.

It is therefore necessary to-provide a cooking face in which thistendency to disintegrate is overcome. To this end, it is provided thatthe cement shall assume the-form of a large number of individual smallpieces within the channel 4. These pieces are separated by minute,almost microscopic cracks. These cracks are so small that they are notreadily visible, and special methods must be employedto make them easilyvisible. Fig. 2 shows a portion of the cooking face with the minutecracks greatly exaggerated as to visibility of the same. In order toprovide the cement in the form of a mosaic of small pieces.

it is preferred to use a cement composed of refractory oxides orsilicates of zirconiumbonded with phosphoric acid. This cement has aninitial plasticity which permits accommodation to theforms of cement aresuitable, a specific example may comprise (by weight) parts of zircongranular, 20 parts of zircon,milled, 8 parts of phosphoric acid(commercial, syrupy), and 8 parts of water. It will be realized that acold setting cement with a proper shrinking on setting might be employedto the same end; however, a

the above-mentioned cement is preferred as it possesses desirableelectrical, thermal, and mechanical properties. It will be obvious thatthe cracks'separating .the cement pieces, formed in this manner, arevery irregular. Further, the separation of the pieces is of an order ofmagnitude equal to or less than the surface irregularities between thepieces themselves and betweenthe pieces and the ribs. locked firmly toone another, to the ribs and to the heating element. Since the cement iscomposed of a large number of small pieces, each piece may reactseparately to thermal shock. This The pieces are thus.

bodiment shown in Fig. l.

. integrate even under extreme conditions, such as,

" for example, whenthe heating element is initially heated at-four timesits normal energization (for flash operation as described in Patent No.2,207,634), or when ice water is poured over the face in red hotcondition. This type of face is also mechanically strong and canstandQgreat mechanical abuse without disintegration. By the provision ofthis strong, flexible, flat face there s obtained a strong, fast, andefllcient ceramic cookchannel as illustrated. At theouter periphery ofthe support, there are provided a plurality of deep thick rib sections22 (see Figs. 5 and 7) below the notches l 8 and 19. At the innerperiphing unit embodying the advantagesset forth I hereinbef ore.

may be seen only with great difliculty as related 'hereinbefore. v

In Figs. 3 to 7 there is shown a preferred form of the cooking unit, inwhich the support structure imposes a smaller thermal load on theheating element and therefore a larger percentageof heat is directedupward to the cooking'surface, with consequent increase in speed andefliciency of the unit. Referring to Figs. 3 and 6, a heating face isprovided, similar in most respects to that shown in the previousembodiment. Ribs. It, channel ll (of spiral form), heating element l2,and cement l3 areiprovided. The various dimensions are similar to thoseindicated for the em- 'The same type of flexible face is provided asprovided in the previous embodiment.

The appearance of the will; is in no way impaired by the mosaic ofcracks, as they cry of the support, there is provided a relatively thinand shallow rib 24 which has notches 25 therein located angularly incorrespondence to the notcnes 23 and disposed immediately below thenotches l6. Extending between the notches 23 and 25 are radial lines 26of reduced floor thickness, the purpose of which will be explainedpresently. These lines underlie the surface score lines 20 previouslymentioned. Between successive lines 26 are radial ribs 21 and 28extending between the rib section 22 and rib 2| and between rib 2| andrib 24 respectively. it will be seen that the lines of reduced floorthickness and the associated notches divide the support into sectors,each of which has a strong supporting rib structure and all of thesesectors are firmly connected together by the continuous rib 2!.

A drastic reduction in the mass of the supporting structure such as thatcontemplated will inevitably cause the structure to crack. If the unitwere to crack in only one or two places, as would occur if otherprovisions were not made, the cracks would become quite large. Theopening and closing of these cracks as the unit heats and cools wouldsubject the portions of the element l2 at the crack to repeatedreversals of Annular ribs l4 and it are provided on the I ment l2 andribs ID. This reduces the electrical leakage of the unit thus increasingthe safety to an erator, and it provides an improved thermal p h m thecooking surface. the grinding of cement fiat may be easily accomplishedwhile this is still in a" somewhat plastic condition, before firing.This is more easily accomplished than the simultaneous grinding of ribsand cement, since the ribs have already been' hardened at this stage andare bound to possess irregularities'of manufacture. Rib i5 has locatedtherein a'plurality of notches l6 (see Fig, 4). Rim l1 and rib it havecorresponding notches l8 and I8 therein. Radial lines 20 (see Fig. 3)

. are scored in the cement and extend over the face of the unit betweenthe inner. and outer notches for a purposeto be described later.

In order to reduce the mass of the supporting brick and thereby increasethe speed and eflicien'cy of the cooking unit, there is provided asupport structure in which a large part of thearea beneath the cookingsurface is reduced to a minimum thickness; In a unit with dimensions asset forth above, the floor thickness beneath the coil l2 may be only0.03 inch. In order .toprovide In addition,

stress, thus causing crystallization of the wire and consequent earlyburnout of the element. Also, the crack might become so large as toexpose parts of the element l2 presenting a hazard to an operator.Further still, such a crack would be very unsightly in appearance. Forthese reasons, it is provided that the unit shall crack into a largenumber of segments, and with this end in view, predetermined parts ofthe unit are weakened so that cracking occurs at these predeterminedlocations. It is for this reason that the above-described lines andnotches are provided. Since the unit is weakened at the radial lines andassociated notches, it will crack when it is heated and cooled. However,it will be noted that thisresults in the cracking ofthe face of the unitinto a large numberof segments (twelve in the drawings). Each of thesesegments is still firmly attached to the rib 2i. The small cracks andthe expansion chamber about the wire (due to burning of enamel) preventcrystallization and consequent burnout of the element l2. In otherwords, the

wire is not held'iirmly on either side of a crack and subjected toexpansion and contraction forces, which would impose stresses, on thewire at one point. Rather thewire is free to move in the expansion spaceso that the stress at a crack is distributed over an appreciable lengthof the wire. Moreover the stresses are distributed over the entire faceof the unit by virtue of the large number of the cracks.

In order to provide further support for the unit, and to adapt it formounting in a range, there is provided an encasing cup 29 (Figs, 3 and6) of suitable material, ceramic or metal being adequate strength forthe support,'the rib structare-shown in Figs. 5 and 6 is provided. Adeep thick rib 2i extends beneath the cooking face and substantiallyfollows one of the spires of the spiral preferred. Around the peripheryof cup 25 is a l n e 30 having a downwardly bent lip, which fioorthereof. (lenient 3i should'have good electrical insulating propertiesso as to minimize electrical leakage from the unit to the cup. The cupis protected from direct radiation from the unit by expanded orexfoliated mica 32, or other suitreached. It will be evident that thepaint as initially prevents such electrical connection beable heatinsulating material. If desired, the material 32 could be omitted and inlieu thereof suitable reflecting means may be provided to reducedownward transfer of heat. For example the cup itself may be reflectingor an additional reflector may be used.

The ends of the heating elements 62 extend downward through holes in thesupport and have s lead Wires 33 connected thereto. These lead wires goto terminals 3 on terminal block 35 which is J fixedly mounted on cup 23as shown in Fig. 3. Some of the lead wires 33 originate at aconsiderable distance from the terminal to which they connect. Notches36 (see Fig. 5) are provided in the ribs 2? so that the lead wires 33may pass throughthe ribs 2i above the surface of the floor of cup 29.The lead wires 33,'passing through the notches 3d are secured therein bycement or other suitable means. I In many instances, in cookingoperations, it is desirable to have a low heat for slow cooking.

For this purpose there is provided an auxiliary heating. element 37 (seeFig. 6). This element might be used, for example, in a cooking systemsuch as is disclosed in the copending application of J W. Myers, SerialNo. 380,341 filed February 24, 1941. A channel 38 is provided directlyabove rib 2 l, which is deeper than the remaining channel ii. Theelement 3l is embedded in this channel below themain heating element E2.The element 3? is preferably of highresistance so as to produce a lowwattage.

' In Figs. 8 to there are disclosed three forms 1 of the terminalstructure (on greatly enlarged scale) employed in the heating unit ofFigs. 3 to '7, all embodying the same basic principles and differingonly in the nature of ceramic support at the various points and thebending of the lead wire. With reference to Fig. 5, Fig. 8 illustratesthe lead wire connection at 39; Fig. 9 illustrates of the lead wire 33is inserted through a substantial number of the end convolutions of theheating element l2, and the lead wire extends for a short distance inthe channel in which the heating element is disposed. Both the heatingelement and thelead wire extend through the opening 42 immediatelyadjacent one of the ribs 21. The descending convolutions or spires ofthe heatthe lead wire connection at points 40; and Fig. 10

illustrates the lead wire connection at M.

' Referring specifically to Fig. 8, the end portion.

ing element are widely spaced in order to obtain a desired thermalgradient along the joint or connection. The end portion of the heatingelement is brazed to the lead wire at 43, thus effecting I oodelectrical connection. Since the brazing operation burns off the enamelon the end portion of the heating coil, a layer of paint 44 is appliedto the entire joint, as illustrated. Like the enamel, this paint burnson when the unit is first used, thus providing expansion space withinthe embedding cement I3. This space is very important as it preventsfractureof the heating element, particularly at the bend, due torepeated expansion and contraction.

The presence of the relatively high mass lead wire 33 results in coolingof the adjacent spires of the heating element.l2. However, no electricalconnection is made between the lead wire tween element 52 and lead wire33. When the unit is heated, the burning away of the paint leaves aspace between the spires of element l2 and the lead 33, and oxide of themetals of the wire forms in this space, which oxide has a suffioientlyhigh dielectric strength to prevent electrical connection between thespires of element l2 and the lead 33 at the highest voltage developedbetween them. However, a very good connection is made between the spiresof element l2 and the lead 33 at the braze 63. It will be appreciatedthat the braze d3 does not corrode a preciably, as it. is maintained ata comparatively low temperature, because of its own relatively highmass, the proximity of the relatively cool ceramic back'structure andthe reduction of heat which has occurred along the spires of element 82,due to the wide spacing thereof. The paint 6d prevents adherence of thebrazed joint 33 to the cement 53 (as well as providing expansion space).This is important, as it allows free, vertical movement of thebrazedjoint under heating. If the braze 63 did adhere to the cement l3,fracture of the wire might occur due to the stress set up by heating;and the movement of the lead wire 33 upward might exert a suihcientforce to break down the insulatling-oxide layer between the lead 33 andthe element l2. v

The above described joint or connection is made prior to the insertionof the heating element l2 in its channel. The lead wire ,is'theninserted downward through the opening 52 until the joint or connectionisdisposed as illustrated.

Such a lead wire connection eliminates burno dt and has even longer lifethan the heating element. itself. By this connection the temperature ofthe heating coil-is gradually reduced to the temperature of the'lea'dwire, and crystallization due to reversals of stresses is prevented.

,The lead wire connection in Fig. 9 is similar to that of Fig. 8, but inthis instance the connection extends through a relatively long opening43 in rib 2!. In this instance the' layer of paint 44 plays an evengreater part because the cement l3 runsdown the long opening aconsiderable distance. While Fig. 9 illustrates the connection for oneof the main heating elements at one of the points 40 (see Fig.5) {theconnections for the auxiliary heating element are similar. l

The lead wire connection in Fig. 10 is also similar to that of Fig. 8,but in this instance the g 1. In an electrical cooking unit, a ceramicsupport structure carrying a shallow cooking race, a deep supporting riblocated beneath said face, a channel formed in said face, a portion ofsaid channel above said rib bein'g deeper than ribs but is securelylocked the remainder of said channel, an electrical of said channel,another electrical heating element disposed in said channel andoverlying said first-named heating element in said deeper portion, andheat-conducting insulating cement embedding saidheating elements,thereby to fix said heating elements in positionand to secure the sameto said support structure.

2. In a surface cooking unit for electric ranges or the like, arefractory support having shallow and narrow circular channels in itsupper portion defined by thin up-standing ribs, said channels and saidribs being of such dimensions radially of the unit that there are notless than four channels per inch in a radial direction, a small-diameterhelical heating coil disposed within and substantially filling saidchannels, and a thermally-conducting electrical-insulating cementfilling said channels and forming a thin working face, said cementhaving a coemcient of contraction such that it develops irregular minutecracks which divide the cement face into a plurality of pieces, wherebythe cement face, when rapidly heated or cooled, cannot exert adisrupting force on said support and the thin to the support by theclosely spaced ribs against displacement from either thermal ormechanical shock.

3. A cooking unit as defined in claim 2, where- 7. A cooking unit asdefined in claim 2, wherein the ribs separating said channels areapproximately one-sixteenth inch in width, the heating I coil has anoutside diameter of approximately in the working face of the unit isweakened along radial lines to cause said face to divide into a numberof predetermined sections.

4. A cooking unit as defined in claim 2, wherein said support has a thinfloor beneath said coil, and supporting ribs extending downward fromsaid floor.

5. A cooking unit as defined in claim 2, wherein said support has a thinfloor beneath said coil, and supporting ribs extending downward fromsaid floor, and wherein the thin floor is weakened along radial lines tocause the entire upper portion of the unit to divide into a number ofpredetermined sections.

6. A cooking unit as defined in claim 2, where in said support has athin floor of approximately 0.03 inch thickness beneath said coil, andsupporting ribs extending downward from said floor,

and wherein the working face and the thin floor are weakened alongradial lines to cause the entire upper portion of the unit to divideinto a number of predetermined sections.

one-tenth inch, the channels are only slightly wider than the coil, andthe depth of the cement above the coil is approximately 0.040 inch.

8. In a surface cooking unit for electric.ranges or the like, a circularrefractory support having shallow and narrow circular channels initsupper portion defined by thin up-standing ribs, a small diameter helicalheating coil disposed within and substantially filling said channels, athermally conducting electrically insulating cement filling saidchannels and forming a thin working face, said supporting structurebeing weakened along radial lines to cause the working face to divideinto a number of predetermined sections to permit release of stress insaid unit in -funi'forrnly distributed manner, said Heating coil havingbeen initially provided with a coating of organic material, which onvolatilization with heat will leave a small space surrounding the wireof the helix, whereby said coil will not be dangerously stressed at thesaid radial cracks when the working face divides itself into the saidsections.

9. In a surface cooking unit for electric ranges or the like, a strongrefractory support having a plurality of thin closely-spaced ribsintegral with its upper face and defining narrow channels thereon, ahelical wire heating element disposed in and substantially filling thechannel width, the outer diameter of said heating coil being aboutone-eighth of an inch, a refractory thermal conducting electricinsulating cement filling the channel spaces around and above saidheating element and forming a thin surface layer above the coils,whereby a practical minimum of electrical insulation is provided overand between said coils, and the center of heat'of said heating elementis disposed near the cooking surface of the unit, the cement having sucha coelficient of contraction that it will develop minute cracks whichdivide the surface into a'plurality of small areas, whereby the cementface when rapidly heated cannot exert a disrupting force on said supportand the thin channel ribs, and the ocment filling and upper layer aresecurely anchored to said refractory support by the walls of saidchannels.

JOSEPH W. MYERS. STEPHEN J ROESCl-l.

